Imtiaz Ali primarily filmed Rockstar in reverse order starting with the long haired final look of Ranbir and moving backward to the clean cut college scenes.
This was done to maintain the natural hair continuity of Ranbir Kapoor and avoid using a fake wig.
Imitiaz wanted Ranbir’s hair growth and look to be completely real rather than artificial. So the scenes where his hair were the longest were shot first which locked-in his heavy intense look without risking continuity issues.
However, it did not negatively affect Ranbir's performance. In fact, filming the intense long haired ending beforehand allowed him to embody the broken mature character of “Jordan” early on before transitioning backward into the innocent college student “Janardhan”.
Instead, Ranbir found the process to be sheer fun and interesting where he had to “unlearn” the intense rockstar persona to play the innocent college student last.
Ranbir stayed in the character of Jordan off-screen. He kept to himself and even avoided smiling in public till the time of the transition.
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